The prevalence of disruptive behavior in children with mental retardation (MR) is substantially higher than in children without MR. Treatment of disruptive behavior in young children with MR is particularly critical because in addition to the stability of these behaviors without treatment, disruptive behavior interferes with their participation in many therapies important to their developmental progress, such as speech, physical, or occupational therapy. Parent training interventions have been reported with children with MR, but no controlled trials of parent training for disruptive behavior in this population have been reported. Parent-Child Interaction Therapy (PCIT) is an evidenced-based parent-training program for preschool-age children with disruptive behaviors that have been used clinically to treat disruptive behavior in children with MR, and the PI has demonstrated the effectiveness of PCIT with a 3-year-old boy with MR and Oppositional Defiant Disorder. The specific aims of this study are to evaluate the effectiveness and generalization of PCIT with families of children with mild and moderate MR and disruptive behavior by comparing a PCIT Immediate Treatment (IT) group with a Waitlist (WL) control group. It is hypothesized that in comparison to families in the WL group, the IT group will show after treatment: (a) significantly fewer child disruptive behaviors as rated by parents and teachers; (b) improved scores on a parent self-report measure of parenting stress; (c) greater child compliance during observed parent-child interactions; and (d) more positive and effective parenting behaviors during parent-child interactions.